This invention generally relates to the manufacture of powder metallurgical products and is specifically directed to an improvement in the techniques used in degassing or decontaminating the powder or porous compacts made from such powder.
The degassing or decontamination of powdered metals to remove volatile or gaseous contaminants such as water vapor and hydrogen is necessary when forming high quality powder metallurgical products. The surfaces of powdered metal particles usually contain both chemically and physically combined moisture and other gaseous or volatile contaminants which can cause internal porosity during subsequent high temperature processing and consequently degrade the ultimate wrought powder metallurgical product.
Prior art degassing procedures, particularly for aluminum base powders, generally emphasize a very high temperature treatment to remove both the physically and chemically sorbed water and other contaminants before the final densification of the powder mass. As a general rule, the degassing temperature was higher than any subsequent temperature the powder metallurgical product is exposed to. Usually, during the high temperature degassing treatment the powdered metal or porous compact is also subjected either to a vacuum treatment or flushed with a flowing inert gas to assist in the removal of the gaseous contaminants. However, neither method has been entirely satisfactory.
Evacuation of the powdered mass to the desired low pressure level at a modest temperature necessitated extensive treatment time which adds considerably to the expense and inconvenience in the manufacture of powder metallurgical products. Also, with large volumes of powder or large compacts thereof it is nearly impossible to develop the low pressures necessary for effective decontamination throughout the entire powdered mass. Additionally, exposure to very high temperature during evacuation has been found to agglomerate the alloying constituents present in prealloyed powder particles which negates to a certain extent a major advantage of powder metallurgy, namely, the uniform dispersion of second phase alloy constituents in an extremely fine form throughout the base metal matrix.
Flushing the powder mass with an inert gas can be less time consuming that vacuum treatments but it is often less effective because the gas tends to channel through the powdered mass leaving portions of the powder or compact poorly flushed. Moreover, the flushing procedures with inert gas can leave significant amounts of the inert gas in the pores of the powdered metal body which can eventually cause porosity in the final manufactured product.
Roberts in U.S. Pat. No. 3,954,458 suggested evacuation of the powder to low pressure levels at an intermediate temperature range of about 450.degree. to 850.degree. F (232.degree.-455.degree. C), preferably about 600.degree. to 800.degree. F (316.degree.-427.degree. C), to remove water, hydrogen and other contaminants from powdered aluminum or an unsintered compact thereof so as to avoid agglomeration of the alloying constituents. While the process described is effective in producing high quality, powder metallurgical products, the process did not reduce the length of time necessary to effectively decontaminate the powder or the compact.
Working a powdered mass can significantly reduce the porosity resulting from any gaseous contaminants present but any subsequent exposure to high temperature will tend to regenerate the porosity. With prior decontamination procedures, particularly as applied to aluminum powders, it was very difficult to reduce the contaminant level to the point where essentially no porosity (visible at 1000X magnification) developed after exposure to temperatures above the solidus temperature of the alloy.
It is against this background that the present invention was developed.